I was stumped about this for a few weeks.
We have re-direct enabled for our site. Anybody that comes into an http page will be sent to https.
Piwik was displaying visitors visiting http versions of pages. It listed http about 50% of the time along with https. We checked our code and our htaccess files and everything seemed fine.
Today, it so happened that an employee was browsing our site and we could see them being fed http versions of the site. Going to their actual browser and checking their browsing history showed NO http pages in their browser only https. Piwik incorrectly displayed http in the Live Visitors view and on the dashboard.